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Old 19-06-2008, 06:35   #1
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Quick Discharge - Also How Bad??

With batteries being so expensive these days, I like to preserve mine and make them last as long as possible.

While I understand most aspects of batteries, I am foggy on one little detail:

The rapid discharge.

If I take 4 AHs out of my batteries in the following two scenarios, is there a difference?

1) I remove 4 AH over 1 hour, with a 4Amp load.

2) I remove 4 AH over 1 minute with a 240Amp load.


Is the high amp draw scenario any worse for my batteries than the long, steady low amp draw scenario?

As you can tell, I've never had the luxury of having a microwave working from my battery bank before. It's handy to use it in the AM to heat up breakfast, or after work to heat up leftovers. This helps a lot to keep condensation down from cooking. BUT... I wonder if it's at the price of my battery longevity or not.

Anyone know?
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Old 19-06-2008, 07:59   #2
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Quote:
I remove 4 AH over 1 minute with a 240Amp load.
Probably starts a fire in about 1 minute. 240 amps DC is a whopper thats more like three microwave ovens. I doubt your wiring is rated for 240 amps.

If you don't draw the bank down below 50% and always recharge it back to 100% then you get the longest life with the most power delivered over the life of the battery. If you do it in an hour or all day and at the end of the day you used the same amount nothing is significantly different.

I really don't think it makes a lot of difference if you do it in 1 hour or 1 minute other than the basic capacity of the wire.

That said the fact that you draw down the bank every day means the number of recharge cycles is going to increase over a one month period and thus shorten the life of the batteries because you are using all the DC power. To some extent you only get a fixed number of recharges before they get tired and can't do it any more. They suddenly discharge even faster than before as they age.

Microwave ovens use the most power the fastest. Volts X Amps = Watts Then add some because the inverter is not 100% efficient. You should feel some heat on the inverter when you run the oven on high.

So no, it won't make your batteries die but yes it will shorten the lifespan since you are using battery power. If you did not use the microwave at all it would lengthen the battery life but so would not using any power at all. It's more about when they die vs. if they will.
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Old 19-06-2008, 08:36   #3
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Good post, Paul. Thanks!

In reality, I'm using 90 amps for the microwave. I just used the numbers above in order to have round numbers for the example.

It's good to hear that the speed at which you drain the batteries doesn't have an effect on the life of the battery one way or the other.

But you make a good point that I'm using up the batteries through discharge on a daily basis... hmmm.... need to rethink things a little. Thanks.
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Old 19-06-2008, 09:20   #4
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I agree with Paul. I don't pretend any expert technical knowledge, but my experience suggests that it makes little difference whether you discharge rapidly or slowly - just don't exceed 50% discharge.

FWIW we routinely used 3 high draw AC appliances (for short periods) over the course of 2 years of cruising: coffee maker, small microwave, and hairdryer. I remember the hairdryer was 1500 watts - can't remember the ratings on the others. We started with a new AGM 800 AH bank. Over two years later it had probably lost some of its efficiency, but the difference was not noticeable and they were still working fine.

But here's a question about recharge cycles: If you are a liveaboard cruiser; rely mostly on solar and wind for charging; and you support some reasonable number of ammenities such as refrigeration, lights, fans, music, computer, etc.; aren't your batteries more or less in a continual state of recharge? That was certainly my impression.
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Old 19-06-2008, 09:29   #5
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That's good to hear, Slomotion. I do have a brand new 600AH bank of Trojans, so it's quality.

My max discharge so far (on this foggy, windless, rainy week in Maine) was about 190 amp hours, measured by the Link 10. This is the max I've ever discharged the batteries. I was hoping solar and wind would work, but they wouldn't. Actually had to break out the little toy genset I bought for these conditions.

When I was down south, in FL, I had to use the genset a few times due to heat making the refrigerator stay on a lot and a cloudy week. In the "middle states", all worked very well. Now, up here in the north, there is too much fog. Otherwise, if I could get a few sunny days, the refrigerator barely runs.

Interesting how solar/wind couples with DC refrigeration work in different areas.
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Old 19-06-2008, 10:02   #6
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Sean, every battery maker publishes capacity/discharge charts. And they vary widely from one to the next. Some may claim 500 cycles at 50% discharge depth, others 2000.

And there's no doubt that "any" cycling is eating battery life, but if you are only cycling them to the 30% point or so--that's a very long life. 50%, more significant.

And the heavier the load, the less power the batteries really have, i.e. if they are rated for 100 hours at a 1-amp discharge rate, they may only be rated 4 hours at a 20-amp discharge rate, the heavier load causes physical changes in the battery that lessen the overall capacity during that cycle. Again, each battery maker has specifics for your batteries.

At a certain point, a really heavy load (like an industrial diesel starter motor sucking on a group24 battery) could generate enough heat internally to damage the battery, but one hopes you won't be doing that unless the situation makes the battery expendable anyway.

So, "less is more", but having batteries and not using them, that would be silly. Kinda like dragging around a big lead keel--inside the boat.<g>
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Old 19-06-2008, 11:27   #7
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Originally Posted by hellosailor View Post
Sean, every battery maker publishes capacity/discharge charts. And they vary widely from one to the next. Some may claim 500 cycles at 50% discharge depth, others 2000.

And there's no doubt that "any" cycling is eating battery life, but if you are only cycling them to the 30% point or so--that's a very long life. 50%, more significant.

And the heavier the load, the less power the batteries really have, i.e. if they are rated for 100 hours at a 1-amp discharge rate, they may only be rated 4 hours at a 20-amp discharge rate, the heavier load causes physical changes in the battery that lessen the overall capacity during that cycle. Again, each battery maker has specifics for your batteries.

At a certain point, a really heavy load (like an industrial diesel starter motor sucking on a group24 battery) could generate enough heat internally to damage the battery, but one hopes you won't be doing that unless the situation makes the battery expendable anyway.

So, "less is more", but having batteries and not using them, that would be silly. Kinda like dragging around a big lead keel--inside the boat.<g>
Thanks. Good post as well.

Luckily, I have the Link 10 which helps approximate the battery's true state of discharge using Peukert's Equation, which factors in the rate of discharge plus a component for how much of your "C" you are using at any moment.

Nice meter, really. So my numbers are pretty accurate.
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Old 19-06-2008, 11:38   #8
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From what I remember charging the submarine's large lead acid battery, hellosailer is right in that a higher discharge rate does slightly reduce the capacity of THAT discharge cycle. I don't think there's a long term issue though. An odd thing that we used to note, from a much bigger scale than a sailboats battery bank, was that if the amps were pulled out quickly, they tended to go back in at a higher amp rate for the same charging voltage. Slow discharges took longer to charge back up.
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Old 19-06-2008, 12:44   #9
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Fishspearit, that makes sense. A fast discharge rate would drain the surface charge (for the most part) and the charge would not have time to even out from the internal areas of the plate. Immediately charging the batteries therefore would only need to replenish the surface charge (for the most part).

The reason it takes so long to bring a deeply discharged battery up to 100% is that it takes a longer time for the internal parts of the plates to chemically change.

However, if a high current load were applied and then the batteries were left for a period of time before being charged, the charge would be replenished at a slower rate.
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